This invention relates to techniques for forming layers of material on a surface of a structure, and more particularly relates to techniques for functionalizing surfaces, including surfaces of carbon-based structures, to enable the formation of layers of material on a structure surface.
In the fabrication of electronic and microelectromechanical systems, a selected coating material or coating process can be found incompatible with a chosen substrate or material structure on which the coating material is to be formed. For example, atomic layer deposition (ALD) of a coating material on a structure surface requires that the structure surface be reactive with the ALD precursor molecules required for producing the selected coating material. But for many material systems, a chosen substrate or material structure may be chemically inert to the requisite ALD precursor molecules for producing a desired material layer on the structure.
For example, this condition is true in general for carbon-based materials such as graphite, graphene, carbon nanotubes, and other fullerene structures. Such materials are characterized by a composition mainly of ordered hexagonal groups of carbon atoms or hexagonal and pentagonal groups of carbon atoms. For example, graphene is a planar layer of carbon atoms that are bound together in a hexagonal network that is one atom thick. Graphite is a bulk structure composed of multiple stacked layers of graphene. Carbon nanotubes can be considered as graphene sheets rolled into nanoscale cylinders. Buckyballs and other so-called fullerene structures can be considered as layers of graphene rolled into nanoscale spheres or other three-dimensional structures.
In general, the carbon surfaces of these structures are chemically inert to a wide range deposition species, including many ALD precursor molecules. As a result, many materials cannot be deposited or otherwise formed on the surfaces of these structures. But the unique physical, electrical, and chemical properties of these carbon-based structures are of particular importance for next-generation electrical and optical applications and for many nanoscale systems.
For example, carbon nanotubes are being employed increasingly for a wide range of nanosystems and nanodevices. The unique electronic structure, exceptional elastic properties, and extremely high aspect ratio all characteristic of carbon nanotubes address many considerations that are currently of interest for nanosystems and nanodevices. In the microfabrication of such carbon nanotube-based systems and devices, it can be desirable or required to deposit one or more layers of material on a nanotube surface. Coaxial coating of a carbon nanotube, such that the longitudinal coaxial surface of the nanotube is substantially fully surrounded by one or more coated layers, can be particularly desirable, for, e.g., providing a coaxially symmetric nanotube structure and material properties. Surround-gate transistors and other device configurations are particularly reliant on such a coaxial coating arrangement. Further, suspended carbon nanotube-based device geometries, often employed for sensing applications, generally are preferably implemented with a coaxially-coated nanotube.
A nanotube wall configuration, electronic structure, and surface properties can all impact the ability to deposit a selected material on the surface of a nanotube. For example, uniform, conformal coating of multi-walled carbon nanotubes (MWNTs) can under some conditions be accomplished by atomic layer deposition (ALD) or by chemical vapor deposition (CVD). ALD allows for the deposition of a wide variety of materials at relatively low processing temperatures with superior thickness precision and high composition uniformity, and therefore is an attractive deposition technique for many applications. By enabling such a high degree of deposition precision, ALD well-addresses carbon nanotube nano-scale geometries and overcomes the limitations of CVD deposition techniques. But for many applications, it is understood that MWNTs can be coated by an ALD process only because MWNTs are characterized by the existence of defects, at nanotube surfaces, that can act as nucleation sites for ALD precursors. MWNT ALD coatings therefore cannot be guaranteed to be reproducible or uniform. Single-walled carbon nanotubes (SWNTs) are characterized by a much more ideal and defect-free surface structure than MWNTs. SWNTs are found to be chemically inert to ALD precursor molecules. As a result, continuous ALD coating onto a SWNT by ALD is not in general conventionally achievable for any process conditions.
Other similar scenarios exist in which a selected coating material or coating process is found incompatible with a chosen nanotube wall structure or other configuration. For example, chemical vapor deposition processes in general cannot be guaranteed to produce a uniform nanotube coaxial coating, and can require plasmas or deposition temperatures that are so high as to impact the electrical or mechanical properties of a nanotube. For many process conditions, SWNTs are chemically inert to CVD precursors at CVD temperatures less than about 400° C. Although physical deposition methods (PVD), such as sputtering or evaporation, can sometimes deposit metals or other materials directly onto SWNTs, such deposits are not conformal, and do not uniformly surround the tubes due to, e.g., the directional nature of the methods. A “conformal” coating on a nanotube is here meant to refer to a coating that wraps completely around the nanotube with uniform thickness on all sides. It is therefore difficult to reliably and benignly make conformal layers of material around a nanotube.
For many applications, it is desirable to coat a nanotube uniformly and conformally with insulating and metallic materials in the formation of an electronic device such as a coaxially-gated nanotube transistor. But in general it can be difficult to uniformly and conformally coat a nanotube, and particularly a SWNT, with selected materials. As explained above, SWNTs are inert to ALD precursors and therefore cannot be coated by an ALD process. CVD and PVD techniques do not reliably produce a thin, uniform and conformal layer on a nanotube. Liquid-chemical deposition methods are known to enable the coating of SWNTs with SiO2, which is a low-κ dielectric, but do not enable the deposition of higher-κ materials on a nanotube.
It has been suggested that to overcome this difficulty, a nanotube surface can first be functionalized to render the surface susceptible to deposition precursor molecules, thereby to enable deposition by a selected technique such as ALD. For example, a liquid-based technique can be employed for functionalizing a SWNT surface by covalent chemical bonding of a functionalization layer to the SWNT surface. The resulting layer provides functional groups, covalently bonded to the nanotube longitudinal sidewall, that are reactive with deposition precursors such as ALD precursor molecules.
While this technique indeed enables uniform ALD film deposition on covalently functionalized SWNT surfaces, the liquid-based process is procedurally tedious and could be impractical for large-scale fabrication scenarios. Furthermore, the covalent nature of the chemical bonding process can in general change the hybridization state of a nanotube, destroying optoelectronic and/or other properties of the nanotube. A post-functionalization heat treatment or other process can be required to recover the initial hybridization state of such a functionalized nanotube. As a result, it has heretofore been impractical to employ covalent functionalization as a means for enabling uniform deposition of a selected material, and particularly an insulating material, on a carbon nanotube.
This condition applies in general across the family of carbon-based materials and structures including graphene, graphite, buckyballs, and fullerene structures other than carbon nanotubes. Covalent functionalization of such carbon-based structures is for many applications detrimental and/or not possible within the limitations of a given microfabrication process sequence. The technical potential for such structures in microelectronic and nanoscale system applications is therefore limited by an inability to form layers of selected materials on the structures.